About 90 million years ago, fires regularly broke out in forests located just 900 km from the South Pole.
At that time, Antarctica did not resemble the ice-covered continent of today. The climate during the Late Cretaceous was much warmer, and the western part of the continent was covered by humid temperate forests. Conifers, ferns and other plants grew as far as the high latitudes. An international team has now found evidence of the fires that swept through these landscapes.
Reconstruction of the temperate swamp forest that occupied western Antarctica about 90 million years ago.
Credit: J. McKay / Alfred-Wegener-Institut — CC BY 4.0.
The researchers studied sediments collected beneath the Amundsen Sea, off western Antarctica. The core contains a sequence dating from approximately 92 to 83 million years ago. It contains tiny pieces of charcoal, as well as fragments larger than 0.5 mm. Their size indicates that some vegetation probably burned close to where these deposits formed.
A single black trace would not be enough to prove repeated fires. The team therefore searched for several independent indicators. They detected substances produced during incomplete combustion, as well as characteristic changes in carbonized plant matter. All these clues tell the same story: the fires returned regularly.
These fires were not necessarily very intense blazes. The condition of the charcoal mainly indicates surface fires, with temperatures estimated at around 370 to 400 °C. Their frequency appears to have increased over the course of the sequence studied. Conifers made up a significant share of the fuel, as also indicated by wood and pollen remains.
To understand this landscape, we must forget the present-day ice sheet. Annual precipitation in this region was estimated at around 1,120 mm. Despite this humidity, seasonal variations could have allowed the vegetation to dry out enough to burn. Lightning is a plausible source of ignition. The known volcanic traces were much farther away and do not fit well with the low temperatures observed.
The change in vegetation is particularly interesting. The environment gradually shifted from a swamp forest to a peatland rich in sphagnum mosses, which can accumulate large quantities of plant matter. Recurrent fires may have limited tree growth and favored a more open landscape rich in ferns. This peatland is the oldest of its kind known in the southern high latitudes.